1. Field of the Invention
The present invention relates to a carrier and a developing agent for electrophotography.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a laser printer or a facsimile making use of electrophotographic techniques, an image is recorded by firstly forming an electrostatic latent image in a developing region on a photoreceptor in which a photosensitive layer is formed on a surface of a core metal, secondly developing the electrostatic latent image in the developing region on the photoreceptor by using a toner contained in a developing agent stocked in a developing device, thirdly transferring the resultant toner image on to a recording material and, then, fixing the thus-transferred toner image on the recording material by being thermally pressed with a fixing roller.
Recently, with a remarkable development of the electrophotographic techniques in line with popularization of computers, for example, durability of various types of devices which constitute the image forming apparatus is enhanced and, then, a longer operation life of the image forming apparatus is realized. Further, in the image forming apparatus, the developing agent to be used for forming the image is required to have a longer operational life, namely, stability of charging characteristics during a period of use thereof. And, then, by allowing the charging characteristics of the developing agent to be stabilized for a long period of time and allowing the toner contained in the developing agent to be imparted with a given charge, image density of the image to be recorded on the recording material is no more fluctuated and, then, a high-quality image having scarce image defect can stably be formed.
As for the developing agent, a two-component developing agent containing a toner particle which contains a binder resin and a coloring agent and develops an electrostatic latent image, and a magnetic carrier which is stirred with the toner particle in the developing device and, then, allows the toner particle to be friction-charged is widely utilized. As for the carrier, such carrier as having a core material and a resin coating layer provided on a surface of the core material is ordinarily utilized.
In order to enhance characteristics of the two-component developing agent, for example, using a developing device constituted such that the two-component developing agent can fully perform the characteristics thereof, selecting an appropriate type of the binder resin to be contained in the toner, selecting an appropriate type of the resin which constitutes a resin coating layer of the carrier, conducting a thermal treatment in order to enhance adhesiveness between the carrier and the resin coating layer, allowing an inorganic particle in the resin coating layer of the carrier or the like has conventionally been performed. Further, it is also known that alumina can be contained in the two-component developing agent as an additive.
For example, a two-component developing agent containing a carrier containing porous iron powder (a core material) having an irregular surface and a resin coating layer formed on a surface of the iron powder, and a toner in which a binder resin contains a fatty acid and/or a metal salt thereof is proposed (refer to, for example, Japanese Unexamined Patent Publication JP-A 7-64330 (1995)). Further, in the paragraph [0047] of JP-A 7-64330, it is described to add alumina to the resin coating layer. However, in a technique described in JP-A 7-64330, such addition of alumina is suggested only for a case in which the resin coating layer is constituted by a fluorocarbon resin. The fluorocarbon resin requires a high temperature of 300 to 400° C. at the time of being thermally fused to the core material and even a small difference of a thermal fusion temperature causes a difference of an electric property. For this reason, in carriers each having the resin coating layer containing the fluorocarbon resin, properties imparting charges to toners are, though being small, different from one another caused by differences of electric properties of fluorocarbon resins. Therefore, in the technique described in JP-A 7-64330, alumina is added for averaging the properties, of carriers, which impart the charges to the toners and alumina is not added for maintaining the properties thereof imparting charges to the toners at a high level for a long period of time. Further, in JP-A 7-64330, an effect of adding alumina to the resin coating layer is not specifically exhibited. Still further, a simple addition of the inorganic particle such as alumina to the resin coating layer generates a so-called carrier-flying phenomenon which causes a damage on a surface of the photoreceptor, an image defect or the like.
Further, a two-component developing agent containing porous iron powder (core material) having irregularity on a surface, and a toner in which a binder resin contains a polyester resin as a main component and, further, an ester amide and/or a salt thereof is proposed (refer to, for example, Japanese Unexamined Patent Publication JP-A 7-134437 (1995)). Further, in the paragraph [0044] of JP-A 7-134437, it is described that alumina can be added to a toner as a flowability improving agent. Namely, in JP-A 7-134437, it is only described that alumina is added as an external additive of the toner. Although alumina which adheres to the surface of the toner as the external additive adheres also to the surface of the carrier by stirring in the developing device, it does not enter inside the resin coating layer of the carrier. Under these conditions, the effect of stabilizing the charge-imparting property of the carrier for a long period of time can not be obtained.
Further, a carrier which contains a core material, an intermediate layer which is formed on a surface of the core material and contains alumina, and a surface layer which is formed on the surface of the intermediate layer and contains a condensate between N-alkoxyalkylated polyamide and a silicone resin containing a silanol group and/or alkoxy group is proposed (refer to, for example, Japanese Unexamined Patent Publication JP-A 2005-49478). However, the carrier described in JP-A 2005-49478 has a two-layer constitution in which a resin coating layer contains an intermediate layer and a surface layer and, further, the surface layer which is most deeply concerned with a charge-imparting property to a toner of the carrier is constituted only with a condensate between a specific polyamide and a silicone resin, without containing alumina. Still further, in a technique of JP-A 2005-49478, as described in the paragraph [0009], an addition of alumina to the intermediate layer is executed for the purpose of enhancing flowability of the carrier by imparting appropriate irregularity on the surface of the carrier. That is, in JP-A 2005-49478, a technical concept that a charge-imparting property of the carrier is stabilized for a long period of time by adding alumina into a resin coating layer is not disclosed. Further, the carrier described in JP-A 2005-49478 not only has a defect in that there is a problem in long-term adhesion between the intermediate layer and the surface layer but also has another defect in that, since it uses a specific synthetic resin and the resin coating layer has a two-layer constitution, production in an industrial scale is difficult.